1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a page printer or a facsimile machine that utilizes an electrophotography process or an electrostatic recording process. The present invention also relates to a process cartridge for use in the above-mentioned image forming apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine, a printer or a facsimile machine that utilizes a transferring type electrophotography process is composed of an electrophotographic photosensitive member (a photosensitive member) generally in the form of a rotary drum, a charging apparatus (performing a charging process) for uniformly charging the photosensitive member with a predetermined polarity and at a predetermined electric potential, an exposure apparatus (performing an exposure process) serving as information writing means for forming an electrostatic latent image on the photosensitive member that has been subjected to the charging process, a developing apparatus (performing a developing process) for visualizing the electrostatic latent image having been formed on the photosensitive member with developer in the form of toner as a developer image (i.e. a toner image), a transferring apparatus (performing a transferring process) for transferring the toner image from the surface of the photosensitive member onto a transferring material such as a paper sheet, a cleaning apparatus (performing a cleaning process) for removing developer (referred to as residual toner or transfer residual toner) remaining more or less on the photosensitive member after the transferring process so as to clean the surface of the photosensitive member, and a fixing apparatus (performing a fixing process) for fixing the toner image on the transferring material. The electrophotography process (including charging, exposure, development, transferring, and cleaning) is repeatedly performed on the photosensitive member for image formation.
Generally, in the cleaning apparatus, there is provided a waste toner recovering container for accommodating the transfer residual toner that has been removed from the surface of the photosensitive member. Therefore, in order to provide an image forming apparatus with a long operating life, it is necessary to design this container to be large. This is not desirable from the viewpoint of downsizing of the apparatus.
In view of the above, there has been developed a cleanerless image forming apparatus that does not have a cleaning apparatus including a waste toner recovering container, in which the transfer residual toner remaining on the photosensitive member after the transferring process is removed and recovered from the surface of the photosensitive member in the developing apparatus by “cleaning simultaneous with developing”.
The cleaning-simultaneous-with-developing process is a process performed during the developing process in the next or succeeding process, namely during the process for developing an electrostatic image after the photosensitive member is successively charged and the electrostatic image is formed by exposure, to recover transfer residual toner remaining on the photosensitive member after transferring. In that process, the transfer residual toner that is present on the area of the surface of the photosensitive member that should not be developed is recovered into the developing apparatus by application of a fog removing bias (i.e. a fog removing potential Vback defined as an electrical potential difference between the DC voltage applied to the developing apparatus and the surface potential of the photosensitive member).
With this process, the transfer residual toner is recovered or collected by the developing apparatus and reused for development of electrostatic latent images in the succeeding processes. Consequently, waste toner can be eliminated and the effort for maintenance of the apparatus can be reduced. In addition, its cleanerless structure is advantageous for downsizing of the image forming apparatus.
Referring to the charging device, recently, a roller-charging-type charger that utilizes a charging roller functioning as a contact charging member have been preferably used in place of a corona charger. The roller-charging-type charger is preferable for its stability in charging. In the roller-charging process, an elastic roller (i.e. a charging roller) having electroconductive properties is brought into pressure contact with a member to be charged and charging of the member to be charged is performed by application of a voltage to the roller.
As to this charging process, there has been proposed and put into practice, for example in Japanese Patent Application Laid-Open No. H63-149669, an AC charging process, in which a voltage consisting of a DC voltage equal to a desired surface potential Vd of the member to be charged and an AV voltage having a peak-to-peak voltage equal to or more than 2×Vth (Vth is a discharge-start voltage or a breakdown or threshold voltage) superimposed on the DC voltage is applied to a contact charging member. With the voltage averaging effect of the AC voltage, the degree of uniformity in the charge can be improved as compared to the DC charging process, so that the electrical potential of the member to be charged is substantially converged to Vd, that is the center of the peaks of the AC voltage.
In the case that the aforementioned contact charging apparatus is used as the charging apparatus for the photosensitive member in the cleanerless image forming apparatus in which transfer residual toner remaining on the photosensitive member after the transferring process is removed and recovered by the leaning simultaneous with developing when the transfer residual toner passes through a charging portion in the form of a contact-nip portion between the photosensitive member and the contact charging apparatus, transfer residual toner, especially reversely charged toner that has a charge polarity reverse to the normal polarity, can adhere to the contact charging apparatus to bring about toner contamination of the contact charging apparatus beyond an acceptable degree, so that a charging error might be caused.
This is because toner as a developer might include toner originally having a charge polarity reverse to the normal polarity mixed therein, though the amount thereof is small. In addition, even toner having a normal charge polarity can be reversed in its polarity by an influence of a transferring bias or separation electric discharge, or the charge amount of the toner can be reduced by static elimination.
As per the above, the transfer residual toner is a mixture of toner having a normal charge polarity, toner having a reversed polarity (i.e. reversely charged toner) and toner having relatively small charge amount. Among these toners, the reversely charged toner and the toner having a small charge amount are likely to adhere to the contact charging apparatus upon passing through the charging portion in the form of the contact-nip portion between the photosensitive member and the contact charging apparatus.
In addition, in order for the transfer residual toner remaining on the photosensitive member to be removed and recovered or collected by the leaning simultaneous with developing in the developing apparatus, it is necessary that the charge polarity of the transfer residual toner on the photosensitive member to be carried through the charging portion to the developing portion be normal and that the transfer residual toner have such a charge amount that allows development of electrostatic latent images on the photosensitive member. In some cases, reversely charged toner and toner having an inappropriate charge amount cannot be removed or recovered from the surface of the photosensitive member to the developing apparatus, so that they sometimes cause image errors.
In view of the above-described situations, the applicant of this patent application has proposed, as disclosed in U.S. Pat. No. 6,421,512, an image forming apparatus provided with developer charge amount control means serving as first developer charging means for charging residual developer disposed upstream of charging means serving as a charger for charging a photosensitive member and residual developer uniformalizing means serving as second developer charging means for charging residual developer disposed upstream of the developer charge amount control means and downstream of a transferring portion.
A DC voltage with normal polarity equal to or larger than the breakdown voltage is applied to the developer charge amount control means, so that transfer residual toner passing through it is charged with the normal polarity by a sufficient electric discharge. Thus, on the occasion that the photosensitive member is subjected to charging over the transfer residual toner in the charging process by the contact charging member, the transfer residual toner that has been charged with the normal polarity does not adhere to the contact charging member. In addition, the residual developer uniformalizing means disperses a patterned transfer residual toner image on the photosensitive member, which is carried from the transferring portion to the developer charge amount control means, over the surface of the photosensitive member to decompose the pattern. The dispersed (or distributed) transfer residual toner is then sufficiently charged with the normal polarity by the developer charge amount control means.
On the other hand, in the case that the developer charge amount control means and the residual developer uniformalizing means have electroconductive fiber brush portions and the transfer residual toner remaining on the photosensitive member is dispersed and distributed by those electroconductive fiber brush portions brought into contact with the photosensitive member, toner is sometimes fused to the surface of the photosensitive member to cause image errors.
The reason why toner is fused to the surface of the photosensitive member is considered as follows. Since the transfer residual toner having passed through the residual developer uniformalizing means and the developer charge amount control means has a high charge amount, through its polarity is normal, it is impossible to recover the transfer residual toner as it is in the developing apparatus. So, an AC voltage is applied to the contact charging apparatus, so that the charge amount of the transfer residual toner is controlled to be an appropriate amount by the static elimination effect of the AC voltage. However, even with such a countermeasure, local excessive charging of the transfer residual toner cannot be prevented in some cases. In that case, the mirroring force of the photosensitive member and the excessively charged transfer residual toner becomes so strong that the toner does not adhere to the contact charging member, cannot be recovered by the developing apparatus and cannot be transferred by the transferring means. As a result, the excessively charged transfer residual toner is fused to the surface of the photosensitive member.
In view of the above, the applicant (inventors) of the present patent application has proposed, as disclosed in Japanese Patent Application Laid-Open No. 2001-215799, an image forming apparatus provided with equipment for moving the developer charge amount control means and the residual developer uniformalizing means back and forth (this movement will be referred to as reciprocating movement hereinafter) in the longitudinal direction of the photosensitive member.
With the reciprocating movement of the developer charge amount control means and the residual developer uniformalizing means in the longitudinal direction of the photosensitive member, it was possible to avoid local excessive charging of the transfer residual toner positively and satisfactory images could be obtained without fusion of toner to the surface of the photosensitive member.
However, in the above-described image forming apparatus utilizing the leaning simultaneous with developing and the roller charging process, in the case that developer charge amount control means equipped with an electroconductive fiber brush portion is provided and the developer charge amount control means and the residual developer uniformalizing means are reciprocated, the following problems arise.
(1) When the reciprocating movement brings end faces (with respect to the axial direction of the photosensitive member) of the developer charge amount control means and the residual developer uniformalizing means to a position inside the range (or length) of the developer carrying portion of the developing apparatus, the following problem might arise.
The developing apparatus causes fog toner (i.e. toner that adheres to a non-image portion in which developer should not adhere) to adhere to the photosensitive member, though the amount of the fog toner is not large. The fog toner includes toner having a charge polarity reverse to the normal polarity and toner that has little charge. In addition, there is transfer residual toner on the photosensitive member after the transferring process, though the amount of the transfer residual toner not large. The transfer residual toner also includes toner having a charge polarity reverse to the normal polarity and toner that has little charge.
Consequently, in the case that end faces of the developer charge amount control means and the residual developer uniformalizing means are brought inside the range of the developer carrying portion of the developing apparatus in the course of the reciprocating movement, when the fog toner and the transfer residual toner are brought into direct contact with the contact charging member without passing through the developer charge amount control means and the residual developer uniformalizing means, such toner will adhere to the contact charging member to bring about toner contamination of the contact charging apparatus beyond an acceptable degree. As a result, a charging error might be caused.
(2) When the reciprocating movement brings end faces of the developer charge amount control means and the residual developer uniformalizing means inside the range of the developer carrying portion of the developing apparatus, toner adheres to the surface of the developer charge amount control means and the residual developer uniformalizing means. When the portion of those means at which toner has adhered is moved to a position outside the range of the developer carrying portion of the developing apparatus, the following problem might arise.
The fog toner or the transfer residual toner that has been brought to a position outside the range of the developer carrying portion of the developing apparatus has been controlled by the developer charge amount control means and the residual developer uniformalizing means to have a normal polarity and an appropriate charge amount. Consequently, such toner will scarcely adhere to the contact charging member, but cannot be recovered by the developing apparatus. As a result, toner adheres to and accumulates on the photosensitive member, so that problems such as toner scattering sometimes occur.